1. Field of the Invention
The present invention relates to a wind-driven power-plant comprising a rotor.
2. Description of Related Art
Known wind-driven power plants, in general, comprise a rotor fitted with three rotor blades and directly or indirectly linked to an electric power generator. The generator in turn may be connected to an electric power grid of a power supplier to feed said wind-driven power into said grid.
In known wind-driven power plants, the rotor blades are angularly adjustable. In this manner the attitude of the rotor blades illustratively impacted by strong winds may be adjusted to lower the wind power input. In this design, these kinds of powerplants adjust the rotor blades in a manner that at least one blade adjustment system is used for all three rotor blades. Known wind-driven power-plants provide at least one rotor blade adjustment system to each rotor blade, each system including one blade adjusting drive with a DC motor coupled through an inverter to the electric grid. As a rule, the relatively high rotational speed of the drive motor is reduced by a step-down gear of very high reduction ratio onto a slowly rotating drive pinion, meshing with a crown gear connected directly to the rotor blade. The rotor blade adjustment system also includes a control unit controlling the blade adjustment drive.
The wind-driven power-plant must be allowed to be decelerated, i.e. lowering its power output and/or being shut down at any time and in the presence of any malfunction to secure it against damage and to make it safe.
As a rule, known wind-driven power-plants are decelerated by rotating the rotor blades into the known “feathered pitch” position. “Feathered pitch” means the rotor blades are rotated away from the wind so that, as in a flag exposed to wind, the effective blade surface impacted by the wind is minimized and hence the wind no longer can apply the power that would be required to maintain rotor rotation and on that account the power-plant shall be stopped entirely or at most shall rotate only very slowly.
Malfunctioning may for instance include electric grid failure. In such a case, the grid manager/operator may prescribe separating the wind-driven power-plant within a given time interval from the grid to protect latter. Separating the wind-driven power-plant from the grid in turn perforce prevents feeding the power-plant output into said grid and critical, excessively high rotor speeds may ensue. For its own protection the power-plant itself must be able to rapidly reduce its rotor speed.
On the other hand, grid failure also entails that the electrical blade adjustment system no longer is electrically fed from it and that the wind-driven power-plant rotor speed no longer can be reduced. In order to nevertheless decelerate the wind-driven power-plant by rotating the rotor blades into the feathered position, a DC voltage source is provided additionally in the rotor blade adjustment systems of known wind-driven power-plants and is directly applied to the blade adjustment drive when the power grid fails, thereby assuring power at all times to the blade adjusting drive.
This known wind-driven power-plant, however, incurs a drawback that because of directly connecting the DC voltage source to the blade adjusting drive, the rotor blades can be rotated only in an uncontrolled manner in a general direction of feathering. Control to accurately set a desired rotor blade angle, or in particular a given rate of rotor blade adjustment, is impossible when wind-driven power-plants of the state of the art discussed do experience malfunctions.
As a result, when the external grid fails, the rotor blades of a wind-driven power-plant must necessarily be rotated into the fully decelerated, i.e. feathered position, whereby in general the power-plant rotational speed will be decelerated fairly abruptly. However, abruptly shutting down a power-plant always entails high stresses that are a factor in power-plant size and hence entail larger power-plant costs.
Moreover, the constrained reduction in power-plant rotational speed always penalizes the power-plant operator economically. Such losses will be especially irksome when illustratively the external grid failure is very brief whereas the speed reduction and the ensuing startup of the power plant generally require considerably more time than the duration of external grid failure.
The objective of the present invention is to so further develop a known wind-driven power-plant as to allow optimized regulation of the rotor-blade angle both in normal operation and, especially, in the presence of various malfunctions, for instance failure of the external grid.